sa is substituted for the s after the reduplicate of ṣaṇ Desiderative of the Causatives of 1. svid 2. svad and 3. sah,
The स substitute of स् debars the cerebral change. In other words, the स of these roots remains unchanged. As सिस्वेदयिषति, सिस्वादयिषति and सिसाहयिषति ॥,